The present invention relates to an apparatus for automatically feeding sheets one by one from a stack of sheets.
Such a sheet feeding apparatus can be used in a facsimile unit, an electrophotographic copying machine and the like. For instance, in an electrophotographic copying machine the above sheet feeding apparatus is sometimes provided as an automatic document feeder which can feed the stacked documents one by one onto a template for projecting an image of a document onto an electrophotographic photosensitive member such as a photosensitive drum.
FIG. 1 shows an embodiment of a known document feeding apparatus comprising a document feed mechanism 5 having a feeding roller 1 rotated in the direction shown by an arrow, a pressing roller 2 arranged opposite to the feeding roller 1 and an arm 4 pivotally supported by a shaft 3. The pressing roller 2 is rotatably supported on the free end of the arm 4. At an upstream position, viewed in the document feed direction, is provided a document separating mechanism 6. A stack of documents 8 to be fed one by one is placed on a document table 7 and is clamped between the feeding roller 1 and the pressing roller 2. When the feeding roller 1 is rotated, the stack of documents 8 is fed toward the document separating mechanism 6 which comprises a separating roller 9 rotated in the direction shown by an arrow, a pressing plate 10 arranged in an inclined fashion with the sheet feed direction and a frictional sheet 11 applied on that surface of the plate 10 which is facing the roller 9. Then the lowermost document is separated from the stack of documents 8 and is further fed forward by means of the feeding rollers 1 and 9, while the remaining documents are retained in position by the separating mechanism 6. In this manner the documents are fed successively from the lowermost one in the stack of documents 8. During this feeding operation, the arm 4 is rotated about the shaft 3 in the counter-clockwise direction in accordance with the decrease in the thickness T of the document stack 8 so that the variation of the thickness T, i.e., the number of documents in the stack 8, can be compensated for.
In such a known apparatus the stack of documents 8 is clamped between the feeding roller 1 and the pressing roller 2 only with a small force due to the gravitational force of the roller 2 and arm 4, and thus between the lowermost document and the feeding roller 1 there is not produced a sufficiently large frictional force for feeding the lowermost document. Particularly when the number of stacked documents 8 is large, there might be produced between the lowermost document and the document table 7, a somewhat large frictional force which is sometimes larger than that produced between the feeding roller 1 and the lowermost document, and thus, the document could not be fed positively and stably.
In order to avoid such a disadvantage it has been proposed, as shown in FIG. 2, to arrange a coiled spring 12 between the arm 4 and a fixed member so as to increase the frictional force generated between the lowermost document and the feeding roller 1. However, in such a known apparatus, when the number of the stacked documents 8 becomes small, there might be produced a serious problem, which will be explained hereinbelow. Such a problem becomes noticeable when the document is a thin sheet such as tracing paper.
When the number of the stacked documents 8 is large as illustrated in FIG. 3A, the sheets are smoothly fed one by one by the feeding and separating mechanisms 5 and 6 even if the documents are thin. However, since the stacked documents are compressed between the feeding roller 1 and the pressing roller 2 by means of the relatively strong restoring force of the spring 12, a large frictional force is produced between the successive documents. Therefore, when the lowermost document 8a is fed by the feeding rollers 1 and 9 as shown in FIG. 3B, the remaining documents above the lowermost document 8a are also advanced together with the lowermost sheet 8a. However, the advance of these documents is suppressed by the friction plate 11 and thus they start bending upward as illustrated in FIG. 3B. The degree of this bending becomes larger as shown in FIGS. 3C and 3D in accordance with the advance of the lowermost document 8a. When the documents are bent to a great extent as shown in FIG. 3D, so-called jamming is produced and the documents are clogged in the feeding apparatus. When such jamming is produced, the documents, i.e., valuable originals, are damaged to an impermissible extent. Further, once the jamming occurs, the operation of the apparatus should be stopped and the jammed documents must be removed carefully. This results in troublesome work and lack of reliability.